class UniqueIdGenerator(object):
"""A dictionary-like class that can be used to assign unique integer IDs to
names.
Usage:
>>> gen = UniqueIdGenerator()
>>> gen["A"]
0
>>> gen["B"]
1
>>> gen["C"]
2
>>> gen["A"] # Retrieving already existing ID
0
>>> len(gen) # Number of already used IDs
3
"""
def __init__(self, id_generator=None):
"""Creates a new unique ID generator. `id_generator` specifies how do we
assign new IDs to elements that do not have an ID yet. If it is `None`,
elements will be assigned integer identifiers starting from 0. If it is
an integer, elements will be assigned identifiers starting from the given
integer. If it is an iterator or generator, its `next` method will be
called every time a new ID is needed."""
if id_generator is None: id_generator = 0
if isinstance(id_generator, int):
import itertools
self._generator = itertools.count(id_generator)
else:
self._generator = id_generator
self._ids = {}
def __getitem__(self, item):
"""Retrieves the ID corresponding to `item`. Generates a new ID for `item`
if it is the first time we request an ID for it."""
try:
return self._ids[item]
except KeyError:
self._ids[item] = self._generator.next()
return self._ids[item]
def __len__(self):
"""Retrieves the number of added elements in this UniqueIDGenerator"""
return len(self._ids)
def reverse_dict(self):
"""Returns the reversed mapping, i.e., the one that maps generated IDs to their
corresponding items"""
return dict((v, k) for k, v in self._ids.iteritems())
def values(self):
"""Returns the list of items added so far. Items are ordered according to
the standard sorting order of their keys, so the values will be exactly
in the same order they were added if the ID generator generates IDs in
ascending order. This hold, for instance, to numeric ID generators that
assign integers starting from a given number."""
return sorted(self._ids.keys(), key = self._ids.__getitem__)